fork() is extremely difficult and slow to implement on Windows. Cygwin managed to pull it off - through no small effort - and even so, their implementation is quite slow. Windows effectively requires you to implement fork() through CreateProcess() and then using IPC to copy state to the new process.
For reasons (presumably efficiency), we emulate fork() on Windows using threads. In fact, perldoc perlfork even freely admits that "if the parent process is killed, all the pseudo-processes are killed as well". If we think this is worth a separate note, we can add another heading that fork-exec does not work on Windows, or add a note to for Portability Caveats section in that doc.
However, this ticket asks "do not emulate fork() using threads", so it is wishlist at best.
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via perlbug: queue: perl5 status: open
https://rt.perl.org/Ticket/Display.html?id=24731